Recently, the request of a highly advanced information society for preventing a blackout is accomplished by a leakage current measurement method which can measure current leakage without blocking a device, in a method of measuring current leakage using an insulation resistance tester in which insulation failure management of a power line or a device accompanies a blackout.
A product which measures AC current and phase difference of a device using the leakage current measurement method includes, for example, a leakage current monitoring device, a leakage current measurement device, a leakage current detection device (circuit breaker) and the like. In addition, a meter which measures AC current of a device includes a power meter, a current meter and the like. These devices or meters perform detection or measurement described below.
Detection or measurement performed using a device or a meter includes measurement of AC current values, and an input current I[A], an active current Ir=I cos θ[A] and a reactive current IL−IC=I sin θ[A] are obtained through the measurement. Other than the AC current values, AC powers are measured, and an apparent power S=VI[VA], an active power P=VI cos θ[W] and a reactive power Q=VI sin θ[Var] are obtained by multiplying a voltage measurement value after measuring the AC current values. In addition, leakage currents are measured, and a total leakage current I0[A], a single phase resistive component leakage current Igr(I0r)=I0 cos θ[A], a single phase capacitive component leakage current Igc(I0c)=I0 sin θ[A], and a three phase Δ-connection resistive component leakage current Igr(I0r)=I0 sin θ/cos 30° [A] are obtained through the measurement.
As is understood from the result of the detection or measurement described above, a leakage current is obtained by multiplying a measured current value I(I0) by a measured phase difference of sin θ or cos θ or by additionally multiplying the obtained leakage current by a voltage measurement value or a coefficient.
In addition, a small-sized, speedy and stable circuit is needed in a circuit breaker in order to measure and monitor the resistive component leakage current. A product of a general circuit breaker is small-sized, and a further smaller circuit is required to be mounted inside the product.
A method of measuring a current value I0 and a phase angle and calculating a leakage current using a microcomputer or a method of obtaining a result by integrating a current value without using a microcomputer is used in conventional circuit breaker. Although the method of measuring a current value I0 and a phase angle and calculating a leakage current using a microcomputer is chiefly used in a monitoring device or a meter, it takes time until a result is obtained due to the operations performed by the microcomputer, and continuous measurement is not allowed. In addition, the microcomputer requires development of operation software using an analog/digital converter of high precision. This is the same for a power meter which measures an active current or the like, as well as a leakage current meter. The method of obtaining a result by integrating a detected current is proposed to be mainly used in a circuit breaker.
Here, conventional leakage current detection methods are disclosed in, for example, reference documents 1 to 3. The reference document 1 proposes a leakage current detection method and a leakage current detection device, in which a total leakage current is detected from three-phase three-wire power lines, and a capacitive component of the leakage current is canceled and only a resistive component of a leakage current is detected by integrating the detected total leakage current throughout a predetermined phase range. The reference document 2 proposes a leakage current detector for detecting a resistive component leakage current flowing through an AC power line, using two half-wave integrators. The reference document 3 proposes a resistive component leakage current detection circuit which determines a leakage current although a frequency of a measured signal is changed.    Patent document 1: Japanese Patent Publication No. 2011-15583.    Patent document 2: International Patent Publication No. 2009/002120.    Patent document 3: Japanese Patent Publication No. Heisei 8-182180.
However, in the leakage current detection methods described in the reference documents 1 to 3, a vector operation is performed after measuring a current value and phase differences of a voltage waveform and a current waveform which are used as a reference, and a value calculated by the vector operation is multiplied by a coefficient or another measurement result, or an integration is performed on the calculated value, and a result of the multiplication or integration is compared with a threshold value. An integration circuit for performing the integration is required to obtain a result by integrating the detected current value, and since a large deviation occurs by using the integration circuit, the deviation needs to be adjusted. In addition, a plurality of integration circuits is needed to simultaneously or continuously measure various types of current values. Since a value of a total leakage current I0 is not correct if the total leakage current I0 exceeds a measurable range, it is preferable to measure the total leakage current together with a resistive component leakage current Igr.
In addition, in a conventional device or meter, the main stream is a method of measuring a current value I0, a phase angle sin θ and a phase angle cos θ and calculating a leakage current using a microcomputer. In this method, a time for stabilizing an output value is required in order to calculate a leakage current using a microcomputer. Since a time is required until an output value is stabilized, an intermittent leakage current which occurs only for a short period of time cannot be measured, and thus measurement of a good response cannot be expected. Therefore, a product capable of performing measurements with a good response is required.
In addition, in the vector operation processed using a microcomputer, a current value and phase differences of a voltage waveform and a current waveform which are used as a reference are measured, and a vector operation or a multiplication is performed on a result of the measurement, and in addition, since the measurement result cannot be obtained every time-delaying half-wave cycle, the burden on the microcomputer is heavy. In the method of using an integration circuit which integrates the detected current value, the three items cannot be simultaneously output since integration is performed after determining a phase difference, and in addition, when a next measurement is performed, a result of the integration needs to be cleared, and thus two integration circuits need to be prepared in order to perform continuous measurements (every half-wave period).